1. Field of the Invention
The present invention relates to a resonant micro-electro-mechanical system with analog driving.
2. Description of the Related Art
As is known, the use of micro-electro-mechanical systems, or MEMS, is increasingly widespread in various sectors of technology and has yielded encouraging results especially in the construction of inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon micro-electro-mechanical structures comprising at least one mass, which is connected to a fixed body (stator) by means of springs and is movable with respect to the stator according to predetermined degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective comb-fingered and mutually facing electrodes, so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors; from this it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the applied force. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. Furthermore, to obtain electromechanical oscillators the frequency response of the inertial MEMS structures is exploited, which is typically of a second order low-pass type. By way of example, FIGS. 1 and 2 show the curve of the magnitude and of the phase of the transfer function between the force applied to the movable mass and its displacement with respect to the stator, in an inertial MEMS structure.
Many MEMS systems (in particular, all electromechanical oscillators and gyroscopes) must envisage driving devices that have the task of maintaining the movable mass in oscillation.
A first known type of solution envisages supplying, in open loop, periodic stresses on the resonance frequency of the MEMS structure. The solution is simple, but also far from effective because the resonance frequency is not known with precision on account of the uneliminable dispersions in the processes of micromachining of semiconductors. Furthermore, the resonance frequency of each individual device can vary over time, for example on account of temperature gradients or, more simply, on account of ageing.
Then, feedback driving circuits have been proposed, which are based upon the use of sigma-delta modulators. Circuits of this type are undoubtedly more effective than the previous ones in stabilizing oscillation of the movable mass at the real resonance frequency and in suppressing any disturbance. However, different stages are necessary for filtering, decimating and further processing the bitstream supplied by the sigma-delta modulator. For this reason, currently available feedback driving circuits are complex to produce, cumbersome and, in practice, costly.